QUETTA, Pakistan -- Pakistani security forces have stormed parts of a hospital occupied by militants in the southwestern city of Quetta.

More than 20 people were killed in violence in Quetta on June 15 that started with the bombing of a bus carrying women students.

At least 11 of the women aboard the bus were killed and some 20 others wounded.

A group of militants then attacked the hospital where the wounded from the bus attack had been taken.

Abdul Wasey, a spokesman for the paramilitary Frontier Corps, said the militants detonated a bomb in the emergency ward killing at least 11 people and wounding 17 others and then took over several areas of the hospital.

Pakistani Interior Minister Chaudry Nisar Ali Khan said security forces killed four militants and captured one more during the raid on the hospital, which freed some 35 hostages.

Pakistan's southwestern Balochistan Province has been plagued by separatist and Islamist violence for years.